This invention relates to a method and apparatus for filtering step waveforms, and more particularly to a circuit for filtering a step waveform of the type which changes value in synchronism with a clock pulse, such as the step waveform output of a digital-to-analog converter.
In most systems which utilize a step waveform, it is necessary, or at least desirable, to filter the step waveform in order to provide a smooth analog signal. Typical examples are waveform simulators of digital-to-analog converters for servo control systems. The filter commonly used is a simple low-pass filter. The trouble with such a simple solution to the problem of smoothing a step waveform is that the low-pass filter tends to introduce high frequency harmonic distortions and undesired phase shifts in the resulting analog signal. What is desired is a filter that is more predictable and more precise for smoothing the output of waveform simulators, digital-to-analog converters, sample-and-hold circuits, CCD (charge coupled device) analog delay lines, and the like.
The application of the present invention to the problem of filtering the output of digital-to-analog converters per se is particularly significant. During the course of the last few years, the importance of analog-to-digital and digital-to-analog converters has increased together with the proliferation of microprocessors and small computers. The reason is that they are necessary interfacing blocks between the chiefly analog nature of the world and the realm of the digital processing that takes place in the microprocessors and small computers.
The digital-to-analog converters can be broadly classified into two main categories, depending on their application. One class of such converters are for use as precision reference sources where their outputs do not change rapidly with time. The other class is for converters used as reproducers of analog waveforms by successions of step functions with considerably higher harmonics in their frequency content. The usual practice is, as noted above, to smooth the outputs through a low-pass filter. However, the resulting waveforms depend on the nature of the low-pass filter, on the rate of change in one clock period which can cause distortions, such as amplitude attenuations and phase shifts, as noted hereinbefore.